Pressure-dependent permeability of Whitby Mudstone

A combination of permeability and ultrasonic velocity measurements allied with image analysis is used to distinguish the primary microstructural controls on effective stress dependent permeability. Permeabilities of cylindrical samples of Whitby Mudstone were measured using the oscillating pore pressure method at confining pressures ranging between 30-95 MPa and pore pressures ranging between 1-80 MPa. The permeability-effective stress relationship is empirically described using a modified effective stress law in terms of confining pressure, pore pressure and a Klinkenberg effect. Measured permeability ranges between 3×10-21 m2 and 2 ×10-19 m2 (3 and 200 nd), and decrease by 1 order of magnitude across the applied effective stress range. Permeability is shown to be more sensitive to changes in pore pressure than changes in confining pressure yielding effective stress coefficients between 1.1 and 2.1. Based on a pore conductivity model which considers the measured changes in acoustic wave velocity and pore volume with pressure, the observed loss of permeability with increasing effective pressure is attributed to the progressive closure of crack-like pores with aspect ratios 0.01 with widths ca. of 10nm associated with grain boundaries. Despite only constituting a fraction of the total porosity, these pores form an interconnected network that significantly enhances permeability at low effective pressures.